Filtration media have been used for the filtration of fine particles from fluids, particularly liquids, for many years. Such filtration media are available in a variety of materials to meet particular filtration requirements. Microporous membranes, such as those described in U.S. Pat. No. 4,340,479, are particularly well-suited to the filtration of fine particulate matter from fluids, especially liquids.
Although microporous membranes possess the ability to remove fine particles, they unfortunately suffer from a lack of mechanical strength. As a result, one or more supportive materials are often mated with a microporous membrane in order to provide the membrane with an adequate degree of mechanical support. This is particularly the case when the microporous membrane is pleated for use in a filtration element.
Support materials of various compositions and structures have been used in conjunction with the microporous membranes, including, e.g., nonwoven fibrous materials such as polyesters and polypropylenes. The particular material used in a filtration medium support material preferably possesses high strength, good edge flow characteristics, and a low pressure drop across the material. Nonwoven fibrous materials prepared from fibers which are at least about 50 microns in diameter provide an exceptional level of performance in each of these areas.
The use of such nonwoven materials as supports for filtration media, especially microporous membranes, however, is not without its problems. In particular, materials prepared from fibers in excess of about 20 microns in diameter typically possess a relatively rough or coarse surface. Thus, when a microporous membrane is mated with such a support material, the support material, due to its roughness or unevenness, can damage and introduce defects into the membrane, particularly when such a membrane is pleated. One example of such a defect is referred to as "coining." Coining occurs when the support material, upon being compressed with a membrane during the preparation of a filtration element, leaves a permanent imprint of its surface in the membrane. The membrane is therefore permanently compressed in the area of the imprint, and this compression results in a local lessening of the membrane's thickness, which in turn reduces the resolution, or titer reduction, of the membrane. In a worst case, a support material will actually penetrate the membrane, thereby rendering the membrane defective and allowing undesirable particles to pass through the filter so as to contaminate the product stream.
It is known that materials which are prepared from fibers which are smaller in diameter offer superior surface smoothness. However, the advantage offered by those materials in the area of membrane compatibility is countered by their increased pressure drop, decreased edge flow characteristics, and decreased column strength which is required to support the pleats of a pleated membrane. Alternatively, such fine-fibered material can be inserted as a cushioning layer between relatively coarse large-fibered material and a microporous membrane; however, this approach decreases the efficiency of the filtration element in several ways. In addition to increasing the pressure drop across the element, the extra layer adds cost and complexity to the element. Moreover, the number of pleats that can be prepared from such a structure are reduced. This reduction in the number of pleats reduces both the surface area available for filtration and the dirt capacity of the filtration element.
Thus, there exists a need for a support material which has an acceptable level of performance in the areas of pressure drop, strength, and edge flow characteristics, but which does not unduly damage a filtration medium, especially a microporous membrane, when mated therewith to form a filtration element. The present invention provides such a support material, and a method for preparing the support material, which provides such advantages while avoiding the aforesaid disadvantages. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.